1. Field of the Invention
This invention relates to binaural hearing aids, and more particularly, to a noise reduction system for use in a binaural hearing aid.
2. Description of Prior Art
Noise reduction, as applied to hearing aids, means the attenuation of undesired signals and the amplification of desired signals. Desired signals are usually speech that the hearing aid user is trying to understand. Undesired signals can be any sounds in the environment which interfere with the principal speaker. These undesired sounds can be other speakers, restaurant clatter, music, traffic noise, etc. There have been three main areas of research in noise reduction as applied to hearing aids: Directional beamforming, spectral subtraction, pitch-based speech enhancement.
The purpose of beamforming in a hearing aid is to create an illusion of "tunnel hearing" in which the listener hears what he is looking at, but does not hear sounds which are coming from other directions. If he looks in the direction of a desired sound--e.g., someone he is speaking to--then other distracting sounds--e.g., other speakers --will be attenuated. A beamformer then separates the desired "online" (line of sight) target signal from the undesired "off-line" jammer signals so that the target can be amplified while the jammer is attenuated.
Researchers have attempted to use beamforming to improve signal-to-noise ratio for hearing aids for a number of years (References 1, 2, 3, 5, 6, 7). Three main approaches have been proposed. The simplest approach is to use purely analog delay-and-sum techniques (2). A more sophisticated approach uses adaptive FIR filter techniques using algorithms, such as the Griffiths-Jim beamformer (1, 3). These adaptive filter techniques require digital signal processing and were originally developed in the context of antenna array beamforming for radar applications (4). Still another approach is motivated from a model of the human binaural hearing system (8, 9). While the first two approaches are time domain approaches, this last approach is a frequency domain approach.
There have been a number of problems associated with all of these approaches to beamforming. The delay-and-sum and adaptive filter approaches have tended to break down in non-anechoic, reverberant listening situations; any real room will have so many acoustic reflections coming off walls and ceilings that the adaptive filters will be largely unable to distinguish between desired sounds coming from the front and undesired sounds coming from other directions. The delay-and-sum and adaptive filter techniques have also required a large (&gt;=8) number of microphone sensors to be effective. This has made it difficult to incorporate these systems into practical hearing aid packages. One package that has been proposed consists of a microphone array across the top of eyeglasses (2).
There are a number of additional problems to the beamforming approach to noise reduction that have not been solved by the above prior art beamformers. If the hearing aid wearer is trying to converse with more than one person at a time, such as in a dinner or cocktail party situation where there are three or four people participating in the conversation, then he must turn his head quickly to look first at one speaker then the next. In addition, if he is looking at one speaker, then he may not be able to tell when a new speaker has begun speaking since speakers other than the one he is looking at are attenuated. Another disadvantage to typical beamforming for noise reduction in hearing aids is the unnatural almost claustrophobic effect which the hearing aid wearer experiences. It limits the usefulness of beamforming to particular high noise situations, such as restaurants and parties, where the desire to communicate overshadows concerns of naturalness. Another problem is audible artifacts, resembling a water fall or babbling brook, which are most noticeable at low signal levels when no one is speaking, or when there are no significant sound sources in the room other than background ambiance: fans, heaters, etc.